The following relates to the semiconductor device arts, light emission arts, light detection arts, and related arts.
Material systems such as the group III-nitride and ZnMgO systems show promise for optical emitters and detectors. Epitaxial growth of such materials provides, in principle, the possibility of constructing complex multilayer structures that optimally position material of an optimal composition (i.e., bandgap) for emitting or absorbing light of a desired wavelength or wavelength range. For example, alloys of GaN and AlN have bandgaps suitable for ultraviolet emission.
In practice, however, such devices are hindered by defects (e.g., dislocations and possibly vacancies or other point defects) introduced during lattice-mismatched heteroepitaxial growth, and by low electrical conductivity due to low impurity doping efficiency. This latter issue is heightened in the case of devices operating in the ultraviolet, since suitably wide bandgap materials for operating in the ultraviolet tend to have low intrinsic electrical conductivity.
One way to counter strain-induced defect formation is through the use of quantum wells, nanowires, or other nanostructures. Quantum wells are sometimes convenient to construct as they fit well into the conventional planar epitaxy paradigm. However, the total thickness of the quantum well and any lattice-mismatched cladding layers must be kept close to or below the critical thickness for nucleation of strain-induced dislocations.
Quantization in two- or three-dimensions is also achievable. For example, epitaxial growth conditions can be identified that promote the spontaneous formation (i.e., self-assembly) of nanostructures such as nanowires (localization in two dimensions) or “quantum dots” (localization in all three dimensions). Instead of relying on self-assembly, a patterned substrate (e.g., a substrate coated with a dielectric layer having drilled holes) can serve as a template for the epitaxial growth.
However, nanostructures raise a further impurity doping efficiency issue, in that the small size of the nanostructure can lead to large statistical variations in doping level, especially at lower doping concentrations. This variation can introduce local variations in bandgap that can trap charge carriers or otherwise impede electrical transport. Such dopant variation may also be driven by systematic factors, such as preferential surface segregation of the dopant atoms leading to a higher density of dopant atoms at the outer perimeter of the nanostructure. Another issue with nanostructures is that the high surface area-to-volume ratio leads to a high effective density of surface states, which are believed to act as material defects that can adversely affect electrical transport and can serve as recombination centers.
These issues can be partially addressed by using nanostructures of larger size, which reduces both statistical variation and the surface area-to-volume ratio. However, if the nanostructure is too large then strain-induced defects again arise, as the nanostructure begins to approximate bulk material.